The present invention refers to lightweight aggregates in the form of discrete, porous and expanded particles, for use in concretes, mortars, building blocks and other similar building elements and, more particularly, it is related to lightweight aggregates having an increased strength accompanied by a considerably low density and a high insolubility in water or steam, as well as very high melting temperatures, and to a process for the manufacture of said aggregates.
It is a well known fact that aggregates have been used for long in the building arts and that in accordance with the knowledge gained from the nature of said aggregates, it has been always known that for high compression strength, high temperature resistance and high adhesion of the concretes, the well recognized construction codes call for the use of only heavy and strong aggregates which are not lightweight in their nature. The use of lightweight porous aggregates has been generally restricted to elements that do not require a very high strength of the material, such as in partitions, coverage slabs and the like, but are generally excluded from structural elements such as struts, girders, beams and the like, particularly in view of the fact that all the lightweight aggregates available in accordance with the prior art, were selected from low strength aggregates such as pumice, lava, slag, fired clay, shale or cinders from coal or coke, and the like. All these materials used for aggregates, as well as some other artificially prepared aggregates on the basis of expanded bentonites and expanded silicates, have left much to desire as to the strength of the material produced with the use thereof, in view of the fact that all such aggregates, in the first place, are available in very few particle sizes, and therefore do not lend themselves for a variety of uses, particularly in view of the fact that all the prior art aggregates are of a small particle size, which required the provision of larger amounts of water whereby the strength of the binders is reduced and cracks are caused in the cured mass when dried.
In the case of the prior art expanded silicate materials, these materials were highly brittle, whereby the particle size could not be increased and, on the other hand, showed a high alkalinity, which caused a chemical reaction with the common binders used and, therefore, the material was furtherly degraded with the consequent loss of strength in the final product.
Therefore, in accordance with the prior art knowledge of different types of aggregates for use in concrete construction, it was considered impossible to obtain mortars or concretes having a high strength and at the same time a very low weight, because these two properties of the concretes and mortars were considered to be exclusive of each other, whereby if a high strength was required, then heavyweight aggregates had to be used in order to provide such a high strength, without any possibility of producing a lightweight building material. On the other hand, when the weight of the material was the dominant characteristic, then the inclusion of lightweight aggregates of the above mentioned nature caused a considerable decrease in the strength of the material produced, whereby it was practically impossible to obtain a concrete or a mortar having both characteristics in conjunction.
Porous aggregates artificially prepared from expanded sodium silicates are also well known, such as is disclosed in U.S. Pat. No. 3,990,901. In the particular case of said patent, such aggregates comprise basically a sodium silicate accompanied by certain oxides to give special properties to the product and a hydraulic binder such as Portland Cement or blast furnace slag, which is subjected to a heating process in two different stages to form pores that reduce the specific weight of the material in an important proportion. However, the alleged lightweight aggregate of U.S. Pat. No. 3,990,901, which contains a hydraulic binder, necessarily results in relatively high specific weights, whereby the fundamental purposes of providing a sufficiently lightweight and yet strong aggregate for use in structural members built with concretes are not achieved and, on the other hand, said addition of a hydraulic binder has as its only objective to avoid the drying steps that otherwise must be effected in the process for preparing the aggregates, but in the prejudice of the low density of the product obtained, whereby the porous aggregates of U.S. Pat. No. 3,990,901 have left much to desire in view of their relatively high density and their unusefulness for various uses, particularly structural uses.
In U.S. application Ser. No. 718,276 filed Aug. 27, 1976 by the same inventor of the instant application, (continued with application Ser. No. 901,873, now U.S. Pat. No. 4,162,166) the production of a lightweight particulate porous aggregate is achieved basically by the addition, to an admixture of sodium silicate, silica and an alkaline earth metal silicate, of boric acid or borax to provide a boric oxide in the final product, which permits the production of particulate porous aggregates having a very low specific weight and yet a high physical strength, whereby such lightweight aggregates solved the majority of the problems extant in connection with the prior art aggregates, including those of U.S. Pat. No. 3,990,901 mentioned above. However, the aggregates obtained by means of the process of the above U.S. Pat. No. 4,162,166, still present serious drawbacks, such as the fact that the starting materials are relatively costly and, on the other hand, such aggregates are not provided with a suitable water or steam insolubility, whereby their usefulness is restricted to structural members that will not be steam-cured, inasmuch as said accelerated type of curing process causes the solubilization of the aggregates and, consequently, the destruction thereof.
Lightweight foamed glass products are disclosed in U.S. Pat. No. 3,793,039 to Rostoker, which have an apparently similar composition as the aggregates of the instant invention. However, Rostoker does not obtain a truly lightweight material and the expansion of the foamed glass disclosed in said patent must be aided by means of a so called cellulating agent (carbon or carbon releasing compounds) to accomplish the goal of reducing the specific weight of the products. Said products, however, reportedly have a specific weight of from about 400 to about 500 kg/m.sup.3 and their mechanical strength undoubtedly must leave much to desire, because the inclusion of the cellulating agent to cause formation of pores at random, adversely affects the structural integrity of the foamed glass, whereby Rostoker has no intention whatsoever and does not even suggest that said products might have use as building aggregates. The main concern of Rostoker is to obtain a foamed glass block having high resistance to thermal shock, hence the reason of having to use a cellulating agent comprising carbon.
The failure of Rostoker to obtain a truly lightweight material having sufficient structural resistance to serve as an aggregate is believed to have been caused by the fact that Rostoker does not realize that the extremely low ratios of M.sub.2 O/B.sub.2 O.sub.3 (with M being an alkali metal) used in the foamed glass, do not produce sufficient pore formation in the material and therefore do not permit the obtention of a low density product with high structual strength, even through the use of the cellulating agent.
Therefore, for long there has been the need, in the construction techniques, of producing a lightweight aggregate which, besides its lightness, will prevent a high compression strength, a low alkalinity which may avoid side reactions with the binders utilized for producing concretes therewith, a high melting temperature and, at the same time, a high degree of insolubility in water or steam in order to render it suitable for use in concretes that will be subjected to the accelerated curing process by means of steam.